Re: Hooting from BPV and a hesitation/flat spot. Help!!! - Saab 900 Bulletin Board - Saabnet.com
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Re: Hooting from BPV and a hesitation/flat spot. Help!!!
Posted by Johann F [Email] (more from Johann F) on Mon, 21 Jan 2002 12:02:31
In Reply to: Hooting from BPV and a hesitation/flat spot. Help!!!, BWG, Mon, 21 Jan 2002 09:44:07
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(There is a post further back with some responses). I had the same symptoms -hoot and flatspot - swapping the orientation so the piston is towards the intake solved both. The important thing to note is when there is considerable vacuum being generated at the mouth of the turbo which will hold the valve open slightly at the negative/positive manifold pressure threshold. This causes boost delay when you accelerate as pressure is being bled off in the initial stages of spoolup - flatspot. When you pull off the throttle the valve resonates before the spring pressure overcomes the vacuum pressure from the vac line on the manifold. You can solve the sound with the Saab solution - a metal weight on the piston prevents the resonance but it won't stop the delay. The higher pressure Viggen valve does not have the weight and hence the hoot. There was somone who tested the performance of the valve in each direction and concluded that it was (fractionaly) better with it in the stock location ( piston to boost) but this was obviously with full throttle flat out tests - you only get this when you are modulating the throttle at higher engine speeds and part throttle - it's to do with the higher relative vacuums at these conditions. I guess if you want the absolute max power for racing then go stock but for general driving go 'against the flow'!
posted by 195.40.13...
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